1. Field of the Invention
The present invention relates to the field of footwear. The invention concerns, more particularly, a footwear sole structure with interchangeable inserts that modify characteristics of the sole structure.
2. Description of Background Art
Conventional articles of athletic footwear include an upper and a sole structure that are specifically designed for use in particular athletic activities. Running shoes, for example, incorporate a lightweight upper that provides the foot with ventilation, thereby decreasing the overall weight of the footwear and removing perspiration from the area surrounding the foot. Sole structures for running shoes are generally designed to provide a high degree of cushioning, which includes ground reaction force attenuation and energy absorption, and may incorporate motion control components for reducing the inward roll of the foot following footstrike. Basketball shoes generally incorporate an upper that protects the ankle from sprains and a sole that provides stability during the commonly executed lunges and quick direction changes. Finally, the sole structures for soccer shoes and football shoes may incorporate spikes that provide a high degree of traction on natural turf playing fields.
Despite the differences between the various footwear styles, sole structures for conventional footwear generally include multiple layers that are referred to as an insole, a midsole, and an outsole. The insole is a thin, cushioning member located adjacent to the foot that enhances footwear comfort. The midsole forms the middle layer of the sole and serves a variety of purposes that include controlling potentially harmful foot motions, such as over pronation; shielding the foot from excessive ground reaction forces; and beneficially utilizing such ground reaction forces for more efficient toe-off. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear resistant material that includes texturing to improve traction.
The primary element of a commonly-employed type of conventional midsole is a resilient, polymer foam material, such as polyurethane or ethylvinylacetate, that extends throughout the length and width of the footwear. In designing the midsole, footwear manufacturers balance the manner in which the midsole provides cushioning with stability. In general, a relatively thick midsole will provide greater cushioning than a relatively thin midsole, but will also have less stability than the relatively thin midsole.
As an alternative, U.S. Pat. Nos. 5,353,523 and 5,343,639 to Kilgore et al., hereby incorporated by reference, discloses an article of athletic footwear with a midsole that includes foam columns placed between semi-rigid upper and lower plates. In general, the foam columns support the entire heel portion of the foot. The heel portion of a conventional article of footwear generally includes a block of foam material and may incorporate fluid-filled bladders, as disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Rudy. In contrast, the heel portion of the footwear disclosed in the Kilgore patents includes foam columns and a void that extends through the columns. Unlike many conventional midsole materials, therefore, the foam columns generally utilize a foam with higher density to provide greater support per unit-volume of foam material.
The performance characteristics of the foam columns disclosed in the Kilgore patents are primarily dependent upon factors that include the dimensional configurations of the columns and the properties of the foam material selected for the columns. By designing the columns to have specific dimensions and foam properties, cushioning and stability of the footwear may be generally tuned to meet the specific demands of the activity for which the footwear is intended to be used. In running shoes, for example, the dimensions and foam properties may be selected to provide greater cushioning. Similarly, the configuration of the columns may also be selected to provide enhanced stability in basketball shoes.
As stated above, cushioning and stability may be generally tuned to meet the specific demands of a particular activity. In general, the dimensions and foam properties will be selected to accommodate specific weights of the wearer, a generally preferred degree of cushioning, a particular activity, a specific ground surface, and a certain degree of motion control. A particular pair of footwear, however, may be purchased and worn by individuals with a wide range of weights that prefer different degrees of cushioning. In addition, the footwear may be used for varying activities on ground surfaces with a wide variety of compliance characteristics. In addition, different wearers may require different degrees of pronation or supination control. Predetermined column dimensions and foam properties may not be sufficient, therefore, to accommodate the requirements and preferences of all wearers that may utilize a particular pair of footwear.